The present invention relates to a vent, system and method for a mold and more particularly to a vent insert, method of making the insert, and system and method for controlling vent flow to and from the mold.
During molding, the atmosphere inside the mold cavity must somehow be vented from the mold cavity to avoid the air in the mold cavity, along with gases from the mold material, from causing defects in the part being molded. If the trapped air is not vented from the mold cavity quickly enough or not vented at all, the finished part can have defects. For example, trapped air can impart knit lines or poor surface cosmetics in the exterior of the finished part. Even worse, gases from the mold material can migrate into the mold material creating porosity or bubbles in the finished part.
Venting also has an impact on how fast parts can be molded. It stands to reason that the faster air and other gases in the mold cavity can be vented from the cavity, the faster the mold cavity can be filled. Conversely, when defects occur because venting is poor or virtually nonexistent, the rate of injecting the hardenable mold material into the cavity is often significantly reduced rather dramatically slowing production. Thus, not only can venting related problems lead to defective finished parts that must be scrapped, these problems can also significantly reduce production, which is also costly.
The reason that venting-related defects occur so frequently is that venting is not well understood and that venting is neither cheap nor easy. For example, vents typically require rather precise machining to create a vent passage or orifice in the mold cavity and a passageway that leads from the orifice away from the cavity. Machining the orifice particularly requires skill because the orifice must be wide enough to allow gas in the cavity to enter but small enough to stop the mold material from entering the orifice and plugging.
Unfortunately, machined-in-place vents, such as core pin vents, ejector pin vents, perimeter vents, and circumference vents, can have problems. One problem that commonly results from machined vents is that volatile gases from the mold material can condense at the narrowest portion of the vent, typically the orifice or passage, plugging the vent. This usually is because the orifice is too narrow and does not increase in cross-sectional size for a great length until it connects to the passageway. As a result, the pressure of the volatile gases is increased in the orifice while it is simultaneously cooled causing some of the gases to condense and plug the vent. If the condensate doesn""t completely plug the vent, it can partially obstruct the vent, rendering the vent effectively useless and requiring the vent to be periodically cleaned. Even if not rendered useless, a partially plugged vent can lead to one or more of the vent-related molding problems previously discussed.
Another problem with machining these vents is that if precise tolerances are not maintained during machining, the resulting vent will not perform properly and may have to be filled and re-machined. For example, if an orifice is off by as little as two one-thousandths of an inch (0.002xe2x80x3), the vent will either be too large and allow mold material to enter the orifice plugging the vent or be too narrow leading to one or more of the above-identified vent-related problem.
In an attempt to overcome these problems, it is known to utilize a one-piece sintered vent insert received in a pocket in the mold that is in gas flow communication with the passage. Unfortunately, the sintering process creates uneven vent surfaces, has pores that are not parallel, and has corners. Any corner or deviation from straight (or smooth) means that the gases being vented do not travel along a straight line causing them to condense.
Another type of vent, a parting line vent, typically suffers from other problems. For example, the flowing molding material can impact against the interior surface adjacent the parting line vent with such force that it can dent or peen the interior surface. This can lead to surface irregularities in the finished part. If the dents extend too far into the vent, the mold material can enter the vent creating unwanted flash that must be trimmed. Of course, any additional operation that must be performed costs money and requires labor, both of which are undesirable. Moreover, because of the peening, that part of the mold must be periodically resurfaced to restore the integrity of the mold surface so future finished parts will have a better surface finish and no flash.
What is needed is an improved vent arrangement.
A mold vent and system that can control flow of a fluid to and from a mold that has a mold cavity into which a hardenable or moldable material is introduced and molded. In one preferred embodiment, the vent of the present invention comprises a plurality of perforations or orifices, each of which preferably is oblong in shape. The vent of the present invention can comprise an insert received in a pocket in the mold cavity or carried by a pin. In another preferred vent embodiment, the vent comprises a reciprocable pin that has clearance between its head and the mold cavity providing a vent passage therebetween. In preferred methods, the system can be used to withdraw atmosphere from the mold cavity, introduce fluid into the mold cavity, or vent atmosphere from the mold cavity before introducing a fluid into the mold cavity.
The fluid can comprise a gas or mixture of gases, such as air, a vapor, such as steam or another vapor, a combination thereof, as well as a liquid. In its preferred embodiment, gas or vapor is directed through the vent. The hardenable or moldable material preferably includes any material, which can be molded or formed in a mold. Examples of such material include plastic, metal, or a metallic material that is introduced into the mold cavity and which is flowable while in the cavity during molding.
Each orifice of a vent is constructed to permit fluid flow of gas or vapor while preventing flow of hardenable material. Each orifice communicates with a vent passage that in turn leads to a vent passageway. So that fluid flowing through an orifice will not condense inside, each orifice has a narrow entrance adjacent the mold cavity that opens in the passage into an expansion chamber downstream. In one preferred embodiment, each orifice is oblong or slot-shaped. As a result of this construction, the venting surface area is increased, which advantageously enables atmosphere within a mold cavity to be more quickly vented permitting faster mold cycle times to be achieved.
The vent can be made as an insert. Such a vent insert communicates with a larger passageway and can be disposed in a pocket or the like. A preferred insert has orifices that are slots. In one preferred embodiment, the insert has at least three slots. Each slot is oblong and narrow in width. Each slot opens in the mold cavity and preferably communicates with a vent passage in the insert that has an enlarged chamber spaced from its opening. This enlarged chamber reduces the pressure of that which is vented from the cavity after it has passed through the narrower opening to prevent condensation at the opening or in the vent passage. In one preferred embodiment, the slot has a width of between 0.005 inches and 0.0005 inches to permit gas and vapor flow while obstructing the hardenable material.
In one preferred insert construction, the insert is a slotted grate. The grate is comprised of two halves, with each grate half having recesses that separate and define fingers. In one preferred assembly, each finger of one grate half is received in a recess in the other grate half and vice versa. At least some adjacent fingers of the assembled vent grate are spaced slightly apart, defining vent slots between them. To help strengthen the vent grate, the end of each finger preferably bears against an adjacent recess endwall that preferably is slanted or inclined so it acts like shelf to support the finger.
In one preferred insert assembly, the grate halves are retained by engagement. For example, one grate half preferably is constructed with at least one key and the other grate half preferably is constructed with at least one keyway that receives the key to retain the grate halves together. In one preferred construction, one grate half has a pair of spaced apart keys and the other grate half has a pair of spaced apart keyways.
If desired, the insert can be press fit or welded to fix it in place. Where the insert is press fit, the pocket has a sidewall that engages the insert to retain the insert in the pocket. Where the insert is welded, the insert is first placed in the pocket and then welded, preferably about its periphery. Where the insert is welded, a high energy density welding process preferably is used. In another preferred arrangement for securing the insert, the sidewall defining the pocket has an inturned portion, preferably a lip, which is deformed around the insert after placing the insert in the pocket.
In one preferred method of making the vent grate of this invention, wafers of discs are cut from stock. Individual wafers or discs are then machined or cut, preferably using a discharge machining process, such as wire EDM, to form the recesses, fingers, keys and keyways. Where it is desired to increase the hardness of the vent grate, each wafer or disc can be hardened before the recesses and fingers are formed.
In another preferred vent, the vent comprises a pin, such as an ejector pin, that has a head smaller than the rest of it. The smaller head provides a clearance between it and the sidewall defining the bore in which the pin is received to permit gas or vapor to flow therebetween to a passage. The clearance preferably is between about 0.005 inches and about 0.0005 inches to enable gas or vapor to flow while preventing flow of hardenable material. To prevent condensation, the head preferably has a necked down region spaced from its head. The necked down region serves as an expansion chamber that reduces the pressure downstream of the mouth of the clearance and prevents condensation.
In a system and method of controlling mold cavity fluid flow, the system includes valving, a source of fluid under pressure, and can include a source of vacuum. The fluid preferably is a gas, a vapor, or a liquid capable of flowing through a mold vent. In one preferred method, a vacuum is created to accelerate removal of atmosphere from within the mold cavity through a vent to help speed molding. In another preferred method, fluid is injected through a vent into the mold cavity after a part has been molded to help separate the part from the mold. In still another preferred method, fluid, such as a foaming agent, is injected through a vent into the hardenable material while it is in a moldable state. In a further method, a heated medium, such as steam, can be introduced into the mold cavity through a vent to heat hardenable material in the cavity. In still a further preferred implementation, atmosphere is removed from the mold cavity through a vent using a vacuum during one part of the mold cycle and fluid is introduced through the vent during another part of the mold cycle.
It is an object of the present invention to produce a vent having greater venting surface area for increasing venting of gas from the mold cavity.
It is another object of the invention to provide a vent capable of bidirectional flow that can be used to introduce gas, vapor, or a fluid into the mold cavity.
It is an advantage of the invention that the vent can be made as a modular vent insert that is lightweight, easy to make, simple to assemble, inexpensive to ship, easy to install, and which is rugged, durable, long-lasting, plug-resistant, and easy to clean.
It is an object of the invention to produce a vent insert that is versatile in that it can be mounted on an ejector pin or disposed in a mold cavity.
It is an object of the invention to produce a vent with orifices having a configuration that obstructs hardenable material from entering the vent and passages configured to prevent condensation in the vent.
It is an advantage of the invention to produce a pin vent that is simple to make, integrally formed from the head of the pin, reliable, rugged and plug-resistant.
It is an object of the invention to provide a system and method of controlling fluid flow to and from a mold cavity.
It is another object of the invention to provide a system and method used to introduce a foaming agent into the mold cavity through a vent.
It is another object of the invention to provide a system and method to increase the rate of heating of hardenable or moldable material.
It is another object of the invention to provide a system and method to accelerate venting.
It is an advantage of the system and method of this invention to selectively accelerate venting in one portion of a mold cavity to influence the flow of the hardenable or moldable material as it is being introduced into the mold cavity.
It is an advantage of the system and method of this invention that a foaming agent can be introduced into a mold cavity through an existing vent thereby obviating costly machining of separate foaming agent passageways into the mold.
It is an advantage of the system and method of this invention that a heating medium can be introduced through an existing vent thereby obviating costly machining of separate heat transfer medium passageways into the mold.
It is an advantage of the system and method of this invention that it is versatile, simple, inexpensive to implement, and easy to use.
It is an advantage of the system and method of the invention that it can be used to vent and introduce a fluid during the same molding cycle.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.